The present invention relates to wireless communication systems, and more particularly, to methods and apparatus for generating CDMA long codes in a wireless communication system.
Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Many communication systems utilizing code division multiple access (CDMA) techniques comply with the IS-95 standard, adopted by the Telecommunication Industry Association (TIA). Under the IS-95 standard, a communication system modulates the information signals with Walsh orthogonal function sequences. To produce corresponding orthogonal information signals, these CDMA systems require that the forward link information signals be transmitted in a synchronized manner. A more detailed discussion of the IS-95 standard is provided in xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993), incorporated by reference herein.
Wireless communications systems, including CDMA systems, are increasingly utilized to transmit voice and data communications. A mobile station desiring a connection to a called party employs an access channel to supply the base station with the dialing number of the called party and an identification of the mobile station (calling party). Thereafter, traffic channels are employed for communications between the calling and the called parties by means of the base station. Generally, users do not want the content of the communication to be publicly available. Without proper cryptographic precautions, however, the communications in a wireless system can be intercepted. Thus, communications in CDMA systems and other wireless systems are typically encrypted to prevent an eavesdropper from accessing the transmission.
FIG. 1 illustrates the manner in which speech signals are encrypted in a conventional CDMA system. As shown in FIG. 1, at the transmitter 100, user data is encrypted by a scrambler/spreader 130 using a short code and a long code produced by a short and a long code generator 110, 120, respectively. At the receiver 150, the encrypted signal is operated on by the inverse process, using the same short and long code to reproduce the original data stream. For a more detailed discussion of speech encryption techniques under the IS-95 standard, see, for example, xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993), incorporated by reference above.
Generally, the short code shown in FIG. 1 is a pseudo-random sequence that differentiates each base station in the CDMA system. Long codes are utilized to differentiate mobile stations. Under the IS-95 standard, the long code is a pseudo-random sequence with a period of 242xe2x88x921 chips that is used for scrambling on the forward (base station to mobile) CDMA channel and for spreading on the reverse (mobile to base station) CDMA channel. Thus, the long code provides limited privacy and uniquely identifies a mobile station on both the forward and reverse traffic channels.
The Telecommunication Industry Association (TIA) has recently adopted a new standard for CDMA networks, referred to as the IS-2000 standard (CDMA-2000). For a detailed discussion of the IS-2000 standard, see, for example, IS-2000.2, xe2x80x9cPhysical Layer Standard for CDMA-2000 Spread Spectrum System,xe2x80x9d Telecommunication Industry Association Doc. No. TIA/EIA/IS-2000, incorporated by reference herein. Among other changes to the IS-95 standard, the IS-2000 standard contemplates base stations and mobile stations that communicate using a plurality of spreading rates. Thus, while IS-95 originally contemplated communications having a spreading rate equal to the chip rate of 1.2288 mega-chips-per-second (Mcps), referred to herein as a 1xc3x97 system, IS-2000 contemplates a number of spreading rates, such as a spreading rate equal to three-times the 1xc3x97 system chip rate, referred to herein as a 3xc3x97 system. To implement a spreading rate equal to three-times the 1xc3x97 system chip rate (3xc3x97), the short and long code sequences may be generated by clocking the code generators 110, 120 at three-times the 1xc3x97 system chip rate (3xc3x971.2288 MHz).
In communication systems that employ more than one spreading rate, however, such as 3xc3x97 and 1xc3x97 spreading rates, there are difficulties in handoffs between the 3xc3x97 and 1xc3x97 sub-systems. Specifically, the long code state for the two systems are different due to the fact that the long code generators are clocked at different chip rates between the two systems. A need therefore exists for a method and apparatus for generating CDMA long codes that can operate in communication systems employing more than one spreading rate. A further need exists for a method and apparatus for generating CDMA long codes that maintains long code state synchronization between two systems employing different spreading rates.
Generally, a long code generator is disclosed that maintains a common long code state between a system with multiple spreading rates, such as a spreading rate equal to the 1xc3x97 chip rate and a spreading rate at a multiple of the 1xc3x97 chip rate (nxc3x97), such as three-times the 1xc3x97 chip rate (3xc3x97). If not otherwise specified herein, the term xe2x80x9cchip ratexe2x80x9d or xe2x80x9csystem chip ratexe2x80x9d refers to the 1xc3x97 chip rate. An nxc3x97 long code generator of the present invention generates n bits for every clock pulse, where the clock operates at the system chip rate. The nxc3x97 long code generator generates n bits for every clock period by having the long code mask value assume n values for each clock period. Thus, the long code mask value is changed at n times the chip rate, while the state of the long code is advanced at the chip rate. Each of the long codes corresponding to the n long code mask values are multiplexed, for example, using an interlacing technique. Thus, the n long code outputs are multiplexed to a single bit stream.
The long code generator includes a conventional (feedback) shift register, AND gate array, modulo-2 adder and clock. In addition, the disclosed long code generator includes a long code mask generator that generates n long code masks for each clock period. Thus, the long code mask generator operates with a chip rate of n-times the system chip rate (nxc3x971.2288 MHz). A long code multiplexer interlaces each of the long codes corresponding to the n long code mask values, for example, by alternating individual bits from each of the n generated long codes.
In one implementation, a clock source operating at three-times the system chip rate drives a divide-by-three counter that cycles through states 00, 01, 10, respectively, at bit positions 38 and 39 of the 42 bit mask. The state of the divide-by-three counter is logically added to the conventional 42-bit long code mask using an adder, to produce three long code masks for each clock period. Thus, for a counter state of 00, the conventional long code mask will be unchanged, for a counter state of 01, the 38th bit of the conventional long code mask will be changed and for a counter state of 10, the 39th bit of the conventional long code mask will be changed. The three long code masks generated for each user must be different from one another, and different from the three long code masks generated for all other users to ensure that the long codes generated will be unique among different users.
The present invention effectively cycles through n distinct long code masks for each clock tick, thereby generating an nxc3x97 rate long code. All the randomness properties necessary for proper CDMA operation are preserved. Furthermore, the present invention is equally applicable on both the forward and reverse links for long code generation. Thus, the present invention maintains long code state synchronization between two systems employing different spreading rates, thereby facilitating handoffs without explicit messaging of state information of the long code generator. The lower clocking rate for the long code generators of the present invention also reduces power consumption. In addition, the present invention allows identical hardware to be used in dual-mode terminals for both 1xc3x97 and nxc3x97 spreading rates.